While working at an employment-service firm in Philadelphia three years ago, Nicole Lee Hallberg—the only woman in the four-person company—knew she was treated differently than her male colleagues. It wasn’t until her male coworker Martin R. Schneider accidentally used her email signature that Schneider realized he had an “invisible advantage.”

In a series of tweets that recently went viral, Schneider shared the story, writing that he accidentally used Hallberg’s email signature but didn’t realize it until a resume-editing client was being “rude, dismissive, ignoring my questions,” and belittling his methods. The company's clients included everyone from people working in STEM (science, technology, engineering, and mathematics) to baristas and zookeepers, Newsweek reported. When Hallberg told Schneider this kind of response was a normal thing, the friends decided to do a two-week experimental email signature swap.

Not surprisingly, Schneider said he was “in hell,” as clients questioned his every suggestion.

“I had a great week; I’m not going to lie,” Hallberg told Newsweek. “People were more receptive, taking me more seriously. They assumed [I] knew what I was doing. I didn’t have to prove it to them.”

Hallberg, who no longer works at the company and has her own business as a freelance copy writer, wrote a post on Medium and said that the sexism didn’t start with the clients. After she survived a rigorous training process, her boss told her, “I wasn’t going to consider hiring any females, but I’m glad I did. You should be proud. I had thousands of applications, but yours stuck out to me and made me decide to give hiring a girl a try.” When Hallberg asked why he hadn’t considered hiring women, he said, “Oh, you know. We’ve always had fun here, and I didn’t want the atmosphere to change.”

After the email signature swap, Hallberg and Schneider approached their boss, and he didn’t believe them, claiming there could’ve been multiple reasons clients reacted differently and they had no way of knowing, Hallberg recalled in her Medium post.

As for Schneider, Hallberg told Newsweek she thinks “it shocked him that it was such a clear cut and immediate change." It was "literally night and day with some of these people," she said.

Hallberg told Newsweek the overwhelming majority of the responses have been positive, but some men still try to justify with “nonsexist” reasons. “So many women have said, ‘Oh my God, thank you for sharing. I have one just like it.’ I don’t know any women who don’t,” she said.